The present invention relates to an automatic cooking control system for a microwave oven which can automatically cook a food contained in heating chamber by utilizing temperature detecting sensors. More specifically the invention relates to an improvement of U.S. patent application, Ser. No. 07/256,964, filed on Oct. 13, 1988, Titled "Automatic Cooking Control System for a Microwave Oven".
According to the conventional system, food contained in a heating chamber is cooked using a method having the steps: detecting the inflow air temperature at the beginning of air flow into a heating chamber; detecting the inflow air temperature at about a ten second period; comparing the present temperature with the temperature detected immediately before; obtaining, if the compared temperatures are equal, the temperature variance of the inflow air by subtracting the inflow air temperature detected at the beginning of actuating a fan from the present inflow air temperature; obtaining the temperature difference between the air flowing in and out by subtracting the present inflow air temperature from the present temperature of the outflow air flowing out of the heating chamber; calculating the temperature increment using the temperature variance of the inflow air and the temperature difference between the flowing out and in air; thereafter executing a first stage heating process by actuating a magnetron until the temperature of the air flowing out of the heating chamber is raised as much as the temperature increment calculated; and executing a second stage heating process for a time period equal to a predetermined constant is multiplied by the first stage heating time. In such a cooking system, there are disadvantages in that the temperature variation and difference and the temperature increment are obtained and calculated when the temperature of the inflow air converges with exterior temperature within a range of 70-80% because the temperature of the inflow air is detected at 10 second intervals it is compared with the temperature of the inflow air detected immediately before, thereby the food occasionally is not cooked correctly. This also results from the resolution of an A/D converter which converts the signal of the temperature detected at a temperature sensor into a digital signal and input it to a microcomputer. The resolution of the A/D converter is generally about 0.5.degree. C., and thus, any the temperature change below 0.5.degree. C. is treated as zero.
Due to the resolution of the A/D converter, even when the temperature difference between the temperature of the inflow air detected at present and the temperature of the inflow air detected just before is substantial, the microcomputer determines that temperature difference is zero if the temperature difference is smaller than the resolution of the A/D converter. Assuming that, for example, the resolution of the A/D converter is 0.5.degree. C., and there is a difference of 0.4.degree. C. between the temperature U.sub.4 of the inflow air detected at the time t.sub.4 and the temperature U.sub.5 of the inflow air detected at the time t.sub.5, as shown in FIG. 1, the microcomputer determines is subject to that the two temperatures U.sub.4 and U.sub.5 are equal, and calculates the temperature variation, difference, and increment in a condition that the inflow air temperature U is converged with the exterior temperature U.sub.N and 70-80%, and then heats the food. It is also noted that the larger the time constant of the temperature sensors, the bigger the error described above.
The above problems can be substantially solved by extending the period for detecting the temperature of the inflow air. When the period for detecting the temperature U of the inflow air is doubled, the temperature variation and difference are determined when the temperature of the inflow air converges with the exterior temperature U.sub.N within a range of about 85-90%. This allows the temperature increment to be calculated so that the food can be more correctly heated. However, such an extension of the period for detecting the temperature U of the inflow air also results in the extension of time for calculating the temperature increment.
In other words, if the temperature difference between the temperatures U.sub.2 and U.sub.3 detected at the time t.sub.2 and t.sub.3 is 0.4.degree. C., as shown in FIG. 2, the microcomputer calculates the temperature increment at the time t.sub.3, but if the temperature difference between the temperatures U.sub.2 and U.sub.3 is 0.5.degree. C., the microcomputer calculates the temperature increment after waiting until at time t.sub.4, and then the food is heated, thereby the initial operation is unnecessarily extended.
As a result, if the detecting period is extended, the reliability of the cooking is promoted, while the time for calculating the temperature increment is also increased.